The present invention relates to variable pressure hydraulic systems, for example, gear ratio selector mechanisms of the type covered by the applicants International Patent Application WO97/05410.
In variable pressure hydraulic systems, hydraulic fluid may be supplied to the system from a hydraulic pressure accumulator, via a solenoid operated proportional flow control valve, which controls pressure in the system by selectively connecting the system to the pressure accumulator or to drain. The solenoid valve is controlled electronically, a pressure transducer being provided in the system to provide an indication as to when the required pressure has been attained and/or the solenoid valve must be controlled to permit fluid into or out of the system.
With such systems, it is necessary to have means to measure the pressure of fluid in the pressure accumulator, so that the pressure accumulator may be charged to the required pressure, if required.
Hitherto, separate pressure transducers have been used to monitor the pressure in the pressure accumulator and the pressure in the hydraulic systems. In WO97/05410, it is proposed that the pressure transducer in the hydraulic gear ratio selector mechanism may be used to monitor pressure in the pressure accumulator, by switching further valves to a null position thereby isolating the gear ratio selector mechanism from the system, while the main valve connects the pressure accumulator to the system. The null position of the further valve means is however a position intermediate of positions which normally connect the actuators to pressure or drain and when conventional flow control valves are used for this purpose, significant leakage problems can be experienced.
In accordance with the present invention the main solenoid proportional flow control valve is modified to provide separate connections between the pressure accumulator and a pressure transducer and the pressure transducer and a hydraulic control system, so that a single pressure transducer may be used to monitor pressure in the hydraulic control system and the pressure accumulator.